The spin labels TEMPO and 5-doxyl stearate have been used to study the temperature dependence of membrane fluidity in normal mouse fibroblasts (3T3 cells) and virus-transformed mouse fibroblasts (SV101-3T3 cells) grown in culture. These measurements have been made on cells grown in standard medium and also on cells grown in lipid-free medium with added exogenous fatty acids. These conditions have been shown to alter the fatty acyl composition of cell membrane phosphatides. The results of the ESR measurements correlate nicely with the earlier agglutination assays (Horwitz, Hatten and Burger, PNAS 71, 3115 (1974). We conclude from these measurements 1) lectin agglutination requires a fluid lipid phase in the vicinity of the lectin receptor, 2) the lipid phase of the cell membranes is heterogeneous with respect to the temperature at which the acyl chains melt, 3) the phase behaviors of the bulk lipid phase of normal and transformed cells are identical, and 4) the bulk membrane fluidities of normal and transformed cells are identical and furthermore appear to be under regulation. (Hatten, Scandella, Horwitz and Burger, J. Biol. Chem. 253, 1972 (1978)). We have also studied membrane fluidity in sea urchin eggs after fertilization to determine whether changes in membrane fluidity might regulate the activation of metabolism which occurs after fertilization. Our results suggest that membrane fluidity might play such a regulatory role (Campisi and Scandella, Science 199, 1336 (1978)).